Studies of aberrations in mitogenic signaling pathway(s) in tumor cells may further our understanding of cancer and may identify novel therapeutic targets. We have recently identified and cloned the complete murine cDNA of Dab2, a 96 kd, mitogen-responsive phosphoprotein, which is one of the two identified mammalian homologs of the Drosophila protein Disabled (dDab). Structural features of Dab2 suggest a signaling role: Dab2 contains a non-SH2 phosphotyrosine interaction domain (PID or PTB) and proline-rich SH3 binding motifs. We have found that Dab2 associates with Grb2 in vivo and in vitro, and have formulated the hypothesis that Dab2 sequesters Grb2 from Sos and thus down-regulates Ras activity. In preliminary studies, we have found that Dab2 exhibited growth suppressing activity. Furthermore, using immunohistochemistry, we have shown that Dab2 is expressed mainly in epithelial/mesothelial cells of the ovarian surface epithelium. However, in a large percentage of epithelial ovarian tumors analyzed, Dab2 expression is lost in surface epithelial-derived tumor cells. Additionally, we have identified several putative dab2 genomic alterations in tumor cells and tissues, supporting the possibility that Dab2 functions as a negative growth regulator, and is inactivated in tumor cells. This study will investigate the role of Dab2 loss in breast and ovarian tumor development and examine the hypothesis that Dab2 is a tumor suppressor by two criteria: 1) the negative growth regulatory properties of Dab2, and 2) its altered expression, mutations, and deletions in tumors. The altered expression, mutations, and deletions of Dab2 will be investigated in breast and ovarian tumor tissues using approaches such as immunostaining, Western and Northern blotting, PCR amplification of genomic DNA, Southern blotting, RT-PCR and sequencing. Inducible and adenovirus-mediated expression of Dab2 will be used to reintroduce Dab2 expression into cancer cells lacking Dab2. Following restoration of Dab2 expression, the cells will be tested in tissue culture for growth, soft-agarose plates for colony formation, and nude mice for tumorigenicity. The results will clarify the role of Dab2 loss in the tumorigenicity of breast and ovarian cancers.